Forms of Phosphorus

Just about everyone working with lakes knows that phosphorus is a key factor in many undesirable features of lakes, most notably algae blooms and. by extension, oxygen and pH fluctuations that impair habitat. But not all phosphorus is created equal. Soluble reactive phosphorus, usually orthophosphate, is the most available form, but is usually only a small fraction of the total phosphorus in any sample. In fact, soluble reactive phosphorus can cycle so fast that its actual measured quantity is not all that important in the interpretation of water quality; low concentrations are normal even in eutrophic lakes.

Total phosphorus is useful as a measure of maximum available phosphorus, but some portion of that total will be refractory, unavailable for uptake by algae. Yet nearly all the empirically determined relationships between phosphorus and other limnological features (e.g., chlorophyll, water clarity) are based on total phosphorus, so measuring total phosphorus is generally an essential part of any lake or tributary monitoring program.

The utility of everything in between soluble reactive and total phosphorus is a matter of some speculation. Empirical work over two decades ago found that total dissolved phosphorus, which is assessed the same way as total phosphorus except that the sample is filtered first, correlates best with algal growth potential. Total dissolved phosphorus is therefore a very useful back-up measurement to go with total phosphorus.

There are other forms of phosphorus that can be measured, and more than one way to measure many of the forms of phosphorus, so there are decisions to be made in any monitoring program that affect results, utility and cost. It is not a simple matter of measuring soluble reactive phosphorus, which is easiest and cheapest to assess. Care should be taken in the choice of phosphorus forms to be measured, the methods for measurement, and the use of resulting data.

Autoanalyzer used for phosphorus measurement